(Click image for full-sized view).
The Brain Spawn were right in saying there was a nexus point at the instant Fry entered the freezer tube. There were 32 people (on-screen) who were present in the cryogenic lab during the 24-hour period of late December 31st to January 1st. Take out all of those who weren’t physically present (e.g. dream people), those who were in the cryogenic lab hours after Fry was frozen and weren’t involved in events (e.g. his parents), and you’re still left with 9 people! (I include alternate reality Fry because he was there, but ceased to exist later). Those nine people can be narrowed down into a small window of about an hour, between 11:55 p.m. or so and 1 a.m. I’d love to see the security footage.
Note: All images, characters and plots are are property of Fox, Comedy Central and Futurama itself. The composite image is mine.